4/17*5/9=5/17*4/9
5/18*7/17=5/17*7/18
5/17*1/6+4/17*5/9+5/18*7/17
=5/17*1/6+5/17*4/9+5/17*7/18
提出5/17
=5/17*(1/6+4/9+7/18)
=5/17*1
=5/17